The purpose of this study is to carry out algorithm and coding training, which is among the 21st century skills, with preservice classroom teachers and to examine the effect of this training on the block-based coding success of the preservice teachers and their self-efficacy perceptions. The design of the research is quasi-experimental design with pre-post test control group, which is one of the quantitative research methods. The research was conducted with 3rd year preservice teachers who were continuing their education at Çanakkale Onsekiz Mart University in the 2019-2020 academic year. The research was conducted with a total of 56 participants, 28 in the experimental group and 28 in the control group. 28 preservice teachers in the experimental group were given 6 weeks of training and an application was made regarding this training. Data were collected through Pretest and Posttest. A pre-post test was applied to measure the change in the participating preservice teachers' self-efficacy perceptions and achievements regarding Block-Based Coding before and after the training. Block-Based Coding Achievement Test and Block-Based Coding Self-Efficacy Perception Scale were used as data collection tools of the research. According to the findings, when the block-based coding achievement test and self-efficacy perception scale data were examined, a statistically significant difference was found between the pre-test and post-test and the preservice teachers’ scores in the experimental group receiving training. It was observed that the preservice teachers’ coding achievement and self-efficacy perception increased. When the research results were examined in general, it was determined that the algorithm and coding training applied within the scope of the research increased the algorithm and coding success, skills and self-efficacy perceptions of the prospective teachers.